1. Field of the Invention
The present invention relates to an imaging device having an electronic zooming function of zooming in an image.
2. Description of Related Art
FIG. 7 is a block diagram showing the structure of a prior art imaging device. In the figure, reference numeral 11 denotes an image sensor unit that converts an input optical signal into an electrical signal, stores it therein, reads the electrical signal stored therein, and outputs it as picture data, and reference numeral 12 denotes an imaging control unit that specifies a predetermined scan region which consists of an effective pixel region and a blanking region within the image sensor unit 11, and controls the scanning timing at which the image sensor unit 11 scans the scan region.
In the figure, reference numeral 13 denotes a frame memory provided with a memory A and a memory B (not shown in the figure) each of which holds one frame of picture data delivered from the image sensor unit 11, reference numeral 14 denotes a magnification specification unit that specifies a magnification for electronic zooming, reference numeral 15 denotes an RW (read write) control unit that writes the picture data delivered from the image sensor unit 11 in the frame memory 13, and reads one frame of the picture data or a part of the picture data held by the frame memory 13 based on the magnification for electronic zooming specified by the magnification specification unit 14, and reference numeral 16 denotes a resolution converter that performs an interpolation processing on the part of the picture data read from the frame memory 13 based on the magnification for electronic zooming specified by the magnification specification unit 14, converts it into picture data having a size of one frame, and delivers the picture data as a video signal.
Next, a description will be made as to the operation of the prior art imaging device. FIG. 8 is a diagram for explaining the scan size specified by the image sensor unit 11 of the imaging control unit 12. As shown in the figure, the scan size consists of an effective pixel region from which the picture data is output and a blanking region from which no picture data is output. FIG. 8 shows the effective pixel region having pa size of 640×480 pixels, which corresponds to a so-called VGA (Variable Graphics Array) and the scan size having a size of 800×600 pixels.
The imaging control unit 12 specifies the predetermined scan region for the image, sensor unit 11, and controls the scanning timing at which the image sensor unit 11 scans the scan region. The image sensor unit 11 converts an optical signal applied thereto into an electrical signal by scanning the scan region specified by the imaging control unit 12 based on the scanning timing from the imaging control unit 12, and stores the electrical signal therein. The image sensor unit 11 then reads the electrical signal that has been stored therein, and outputs it as picture data. FIG. 9 is a diagram showing a way in which the image sensor unit 11 scans the scan region specified by the imaging control unit 12.
First of all, the description will be directed to a case where the magnification specification unit 14 sets the magnification for electronic zooming to 1×. The RW control unit 15 writes one frame of picture data delivered from the image sensor unit 11 in the memory A of the frame memory 13 at a certain timing regardless of the magnification for electronic zooming specified by the magnification specification unit 14. The RW control unit 15 then writes the next frame of the picture data in the memory B of the frame memory 13 at the next timing. Thus, the picture data delivered from the image sensor unit 11 is written in the frame memory 13 on a frame-by-frame basis regardless of the magnification for electronic zooming specified by the magnification specification unit 14.
When writing one frame of the picture data in the memory B of the frame memory 13, the RW control unit 15 reads one frame of the picture data which has been written and held in the memory A based on the 1× magnification for electronic zooming specified by the magnification specification unit 14, and then reads one frame of the picture data held in the memory B of the frame memory 13 at the next timing.
FIG. 10 is a diagram for explaining reading and writing of data from and in the frame memory 13 when the magnification for electronic zooming is set to 1×. Thus, when the magnification for electronic zooming is 1×, the whole of one frame of the picture data held in the memory A or the memory B can be read out. In this way, frames of the picture data are sequentially written in the frame memory 13 one by one, and are read out one by one and are delivered to the resolution converter 16.
The resolution converter 16 outputs frames of the picture data read from the frame memory 13 one by one, without change, as a video signal based on the 1× magnification for electronic zooming specified by the magnification specification unit 14.
Next, a description is given for an example where the magnification specification unit 14 sets the magnification for electronic zooming at 2× for a central part of one frame of the picture data. The RW control unit 15 writes one frame of the picture data delivered from the image sensor unit 11 in the memory A of the frame memory 13 at a certain timing regardless of the magnification for electronic zooming specified by the magnification specification unit 14. The RW control unit 15 then writes the next frame of the picture data in the memory B of the frame memory 13 at the next timing. Thus, frames of the picture data delivered from the image sensor unit 11 are written into the frame memory 13 one by one regardless of the magnification for electronic zooming specified by the magnification specification unit 14.
When writing one frame of the picture data in the second memory B of the frame memory 13, the RW control unit 15 reads the central part having a fourth of one frame of the picture data written and held in the first memory A based on the 2× magnification for electronic zooming specified by the magnification specification unit 14. The RW control unit 15 then reads the central part having a fourth of one frame of the picture data written in the second memory B of the frame memory 13 at the next timing. FIG. 11 is a diagram for explaining reading and writing of data from and in the frame memory 13 when the magnification for electronic zooming is set to 2×. Thus, frames of the picture data are written in the frame memory 13 one by one and the central part having a fourth of each frame of the picture data is read out and is delivered to the resolution converter 16.
The resolution converter 16 performs an interpolation processing based on the 2× magnification for electronic zooming specified by the magnification specification unit 14 so that the central part of each frame of the picture data read from the frame memory 13 is enlarged twice in vertical and horizontal directions. Finally, the resolution converter 16 makes the central part have the same size as that of one frame of the picture data and delivers it as a video signal.
Next, the data rate at which the picture data held in the frame memory 13 is read will be explained. When the magnification for electronic zooming is set to 1×, the RW control unit 15 reads one frame of the picture data held in the frame memory 13 at the same data rate as that at which the picture data is written into the frame memory, and the resolution converter 16 delivers a corresponding video signal at a predetermined frame rate.
On the other hand, when the magnification for electronic zooming is set to 2×, in order to keep the frame rate constant, the RW control unit 15 reads the picture data on the central part held in the frame memory 13 at a fourth of the data rate at the time of writing. This is because the amount of the picture data to be read is reduced to a fourth of the data amount of the picture data written into the frame memory.
Thus, when the magnification for electronic zooming is set to other than 1×, it is necessary to change the data rate at the time of writing and that at the time of reading to keep the frame rate constant, and it is necessary to provide the frame memory 13 with memories A and B for holding two frames of picture data. For example, in the case of VGA, the effective pixel region has a size of 640×480 about 300,000 pixels, so a 2-port RAM having a capacity of about 4.8 Mbits or two single-port RAMs each having a capacity of about 2.4 Mbits, is needed to hold two frames of picture data.
In other words, the prior art imaging device can keep the frame rate constant regardless of the magnification for electronic zooming when delivering the picture data finally enlarged according to the magnification for electronic zooming by changing the data rate at the time of reading the picture data from the frame memory 13 according to the magnification for electronic zooming.
A problem with the prior art imaging device constructed as mentioned above is that there is a necessity to install a large-capacity frame memory 13 in order to keep the frame rate constant when performing electronic zooming, thereby increasing power consumption, the size of the imaging device, and the cost. Particularly, it is difficult to apply such a prior art imaging device to mobile equipment.